In a transistor using a nitride semiconductor, high withstand characteristics are obtained, due to electrical material characteristics of the nitride semiconductor that is a material with a wide band gap, and such a transistor becomes a semiconductor device that replaces a power device using silicon. A nitride semiconductor has polarization resulting from symmetry of the crystal structure thereof, and it is known that a two-dimensional electron gas may be generated at a lamination interface of nitride semiconductor layers with different compositions of constituent elements. Since the two-dimensional electron gas has a high electron mobility, a transistor using the two-dimensional electron gas as a channel can electrically perform a high speed response, and when the transistor is used as a high speed switching device, a high energy conversion efficiency is obtained. The transistor using the nitride semiconductor like this has a high breakdown voltage and high speed responsiveness, and thereby it is expected that the transistor like this is used for a power conversion device such as a power source.
In the laminated structure of the nitride semiconductors, the two-dimensional electron gas generated by polarization is accumulated even in the state in which an external electric field is not applied, and thereby in the transistor using the two-dimensional electron gas as the channel, a threshold voltage thereof becomes a negative voltage. But a transistor with the threshold voltage of a positive value is desired from the viewpoint of improving safety, reducing power consumption, and simplifying constitution circuits. As a technology to raise the threshold voltage, a technology is known in which a concave shape groove is formed in a device only at a portion below a gate electrode so that the two-dimensional electron gas is not generated therein. In addition, a technology is known in which a channel layer is laminated on a back barrier layer to improve the threshold voltage.
For example, when a gallium nitride (GaN) layer is used as the channel layer and an aluminum gallium nitride (AlGaN) layer is used as the back barrier layer, the higher a composition of aluminum (Al) contained in the back barrier layer is, the more the threshold voltage can be improved.
However, when an Al composition of the back barrier layer is made higher, a lattice mismatch rate between the back barrier layer and the channel layer becomes higher. Accordingly, when the channel layer is laminated on the back barrier layer, crystal defect is easy to be introduced into the channel layer, and thereby crystal quality of the channel layer deteriorates.
Specifically, surface flatness of the channel layer falls and the thickness of the channel layer becomes nonuniform. In addition, crystal defect such as threading dislocations is increased in the channel layer. When the crystal quality of the channel layer deteriorates, the electron mobility in the channel layer is decreased, and thereby an ON resistance of the transistor is increased to increase power consumption. Accordingly, a nitride semiconductor device capable of improving the crystal quality of the channel layer and improving the threshold voltage is desired.